A cysteine proteinase inhibitor purified from apple fruit

Acysteine proteinase inhibitor has been purified from immature fruit of Malusdomestica (var. Royal Gala). The Mr of this apple cystatin is estimated to be 10 700 by MALDI–TOF mass spectrometry, 11 300 by SDS–PAGE and 11 000 by gel filtration. It is a relatively strong inhibitor of papain with a Ki value of 0.21 nM and also inhibits ficin and bromelain but not cathepsin B. An amino acid sequence was obtained from a peptide produced by trypsin digestion of the inhibitor. Comparison with other plant sequences shows a high degree of homology with other phytocystatins. As the single cysteine proteinase inhibitor detectable in immature apple fruit (5–8 mm diameter), levels of 83.3 pmol\g FW were determined. In larger fruit (up to 16 mm diameter) significantly less inhibitor was present (6.9 pmol\g FW). Given these low levels, it is postulated that this inhibitor has an endogenous role in apple fruit development rather than one of protection against pest or microbial attack.